Various standard setting bodies, such as the Underwriters Laboratory, require a preignition purge for forced and induced draft furnaces prior to lighting an interrupted or intermittent pilot. The purge sequence must assure a predetermined volume of air moves through the system--typically four air changes. One way of meeting the standards is forcing purge air through the system at a certain rate for a specified period of time; this is the method most often used because it is the easiest to implement.
For example, an acceptable purge sequence is a 30 second purge at an air flow rate equivalent to that provided at rated, high fire input to the burner. In the prior art purge sequence the blower motor is energized at substantially the same time the damper motor is energized in order to open it to its high fire position. The purge interval starts when the damper reaches a predetermined position and continues for a prescribed interval. At the end of this fixed interval, the damper is driven to its low fire position and the pilot is lit.
This prior art purge cycle is satisfactory in meeting safety standards set by the various regulatory bodies. However, for years and years those skilled in the art have failed to recognize that a substantial amount of energy is wasted during the purge cycle.
An object of this invention is to provide an improved purge method which markedly increases the efficiency of the system witout: (a) increasing the cost of the control; and (b) degrading the ability of the system to meet applicable standards.